1. Description of the Prior Art
The production of polymer foam, such as polyurethane, polyisocyanurate and like rigid foams, in the form of continuous bunstock by depositing foam forming mix in a U-shaped mold comprised of a lower conveyor belt and side panels moving in synchronized manner, is well-known in the art. The bunstock so produced is then generally cut to fabricate sheets, boardstock and the like for insulation purposes. In order to avoid substantial waste of useful foam in the cutting of boardstock and the like from the polymer foam bunstock, it is highly desirable that the bunstock be produced with substantially rectangular cross-section.
Various processes and devices have been employed to achieve this end. Illustratively, U.S. Pat. No. 3,655,311 describes the use of a tunnel shaped mold having a rectangular cross-section and having the top formed by a series of plates yieldably mounted so that they could be displaced upwardly if the rising foam exerted pressure thereon in excess of a predetermined level.
U.S. Pat. Nos. 3,965,228 and 3,998,575 describe respectively a process and apparatus for preparing substantially flat top foam bunstock by subjecting the lateral edges of the rising foam to the lifting action of a plurality of endless wires thereby seeking to overcome the drag exerted on said edges by friction between the rising foam and the sidewalls of the U-shaped mold employed to prepare the bunstock. U.S. Pat. Nos. 3,091,811; 3,719,734; 3,751,197; 3,809,512; 3,812,227 and 3,965,228 describe closely related apparatus and processes for achieving the same result using forces exerted by side papers moving at an inclination to the vertical corresponding approximately to the profile of the rising foam.
U.S. Pat. No. 3,768,937 describes a similar process in which air or gas filled elongated tubular sheets are fed along the sides of the rising foam in an otherwise conventional bunstock apparatus and are caused to rise in approximate synchronization with the profile of the rising foam so as to minimize frictional drag on the edges of the rising foam.
U.S. Pat. No. 3,325,823 describes a process in which an effect similar to that in which the side papers are moved upwardly is achieved by moving the side papers in the conventional manner but causing the rising foam to be moved downwardly so that a relative upward force is exerted by the side papers on the edges of the foam.
U.S. Pat. No. 3,984,195 describes a process for producing bunstock with a substantially rectangular cross-section by applying molding pressure on the rising foam by means of a plurality of rollers and the like throughout the major part of its rise profile as well as passing the initially deposited foam mix through a nip roll to achieve uniform thickness in the poured foam prior to rise.
U.S. Pat. No. 3,123,856 discloses a process in which elevated portions are provided in the center of the initial section of the floor of the moving U-shaped mold. The foam in the central portions of the mold accordingly rises initially to a greater height than the foam on the outer edges of the mold. Once the foam moves off the elevated section of the mold and reaches the section having a planar floor, the central portions of the foam subside thereby tending to form a more planar surface on the bunstock than that achieved by using a conventional bunstock molding apparatus.
U.S. Pat. No. 3,887,670 discloses the use in an otherwise conventional bunstock line of a combination of weirs and a pour board which has a profile corresponding to the mirror image of the rise profile of the rising foam whereby the frictional drag on the sides of the foam is minimized and a substantially rectangular cross-section is achieved in the resulting foam bunstock.
More recently, French application No. 74 27210 has described a process in which the rising foam is subjected, in an otherwise conventional foam bunstock apparatus, to the molding action of a fixed plate which straddles the rising foam at a point very close to the beginning of foam rise and which confines the foam to a particular configuration over the majority of the rise profile.
British specification No. 1,465,900, which issued after the present invention was completed, teaches the application of pressure to the rising foam over substantially the whole rise profile by means of a sheet contoured to match said rise profile.
The various processes discussed above generally require relatively extensive modification of conventional foam bunstock apparatus. In addition, they lack the flexibility necessary to deal, without modification, with the production of different types of polymer foam having different rise characteristics and profiles. For example, the rise characteristics of a polyurethane foam forming mix often differ quite dramatically from those of a polyisocyanurate foam forming mix. Bunstock apparatus designed to produce the former type of foam with rectangular cross-section could require extensive modification before being capable of production of the latter type of foam.
We have now developed a process for producing polymer foam bunstock having substantially rectangular cross-section without sacrifice of good structural properties which process can be carried out with only very simple modification of conventional foam bunstock apparatus and which can be readily adapted to producing bunstock from polymer foam mixes having widely varying rise profile characteristics.